The following description is provided to assist the understanding of the reader. None of the information provided is admitted to be prior art to the present technology.
The use of rechargeable lithium-ion batteries in portable electronic equipment such as cell phones, PDAs, laptop computers, and implantable medical devices has continued to increase in recent years. While lithium ion batteries appear to be the battery of the future, there are increasing concerns about the safety, especially for the development of large lithium-ion batteries for Electric Vehicle (EV) and Hybrid Electric Vehicle (HEV) industries.
Aqueous electrolytes cannot be used with lithium batteries due to the reactivity of the active materials with the water. Organic electrolytes exhibit high voltages, have high ion conductivity, high dielectric constants, and low viscosity, however, they also tend to be flammable, toxic, and can react vigorously at elevated temperatures with electrode materials. Under certain conditions, lithium ion batteries may undergo thermal runaway, generating a sharp rise in temperature, and possibly resulting in fire or explosion. Organosilicon-based electrolytes, which are nonvolatile, nonflammable, and nontoxic, have received interest as an alternative electrolyte for lithium ion batteries. Organosilicon-based electrolytes have good ionic conductivities, excellent safety features and show excellent electrochemical and chemical properties in lithium ion batteries. However, shortcomings do exist that limit the commercialization of these silicon-based electrolytes. For example, the relatively low lithium conductivity and lack of a solid electrolyte interphase (SEI) formation capability on the carbon anode based lithium ion cells can be problematic.
When a organic, polar, non-aqueous solvents are used in the electrolytes of lithium ion batteries, an irreversible reaction occurs to form a passivation layer on the surface of the anodes during the initial charging cycles (i.e. the first cycles the battery undergoes). This passivation layer, or SEI, is formed on the surface of the anode. The SEI prevents the degradation of the electrolyte solution during charging and discharging, and acts as an ion tunnel. The SEI film influences the discharge capacity during subsequent cycles and influences several important aspects of battery performance such as cycle life, power capability, and self-discharge rates or shelf life and safety. Physical and chemical properties of the SEI film change according to the salt used in the electrolyte, the concentration of the salt, the composition of the solvent mixture, and other types of additives that may be used. Preferably, the SEI layer formed on the anode surface is relatively thin and of a uniform thickness. If an uneven SEI film is formed, the lithiated graphite in the active materials will react with the electrolyte components and cause decomposition of the electrolyte leading to a loss of active lithium. Accordingly, the irreversible capacity of the active material is increased and the capacity and lifetime of the battery is reduced. In the case of organosilane or organosiloxane-based electrolytes, most traditional electrolyte components, such as lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiDfOB), and vinyl carbonate (VC), cannot form a uniform SEI layer. For example, when LiBOB is employed as an electrolyte additive for organosilane or organosiloxane-based electrolytes, a relatively thick SEI layer is formed on the negative electrode surface due to reductive decomposition of LiBOB on the anode. The formed thick SEI layer increases interfacial impedance inside the cell resulting in poor cyclability and poor power capability. Additionally, a too-thick SEI can't completely suppress the further decomposition of electrolyte on the anode.
Accordingly, there is a desire to develop functional electrolyte additives and materials that can reduce the interfacial resistance and improve the power capabilities of lithium ion batteries.